Much of what you have learned about resonance and series-LC circuits can be applied
directly to parallel-LC circuits. The purpose of the two circuits is the same - to select
a specific frequency and reject all others. XL still equals XC at
resonance. Because the inductor and capacitor are in parallel, however, the circuit has
the basic characteristics of an a.c. parallel circuit. The parallel hookup causes
frequency selection to be accomplished in a different manner. It gives the circuit
different characteristics. The first of these characteristics is the ability to store
energy.

The Characteristics of a Typical Parallel-Resonant Circuit

Look at figure 1-11. In this circuit, as in other parallel circuits, the voltage is the
same across the inductor and capacitor. The currents through the components vary inversely
with their reactances in accordance with Ohm's law. The total current drawn by the circuit
is the vector sum of the two individual component currents. Finally, these two currents, IL
and IC, are 180 degrees out of phase because the effects of L and C are
opposite. There is not a single fact new to you in the above. It is all based on what you
have learned previously about parallel a.c. circuits that contain L and C.

Figure 1-11. - Curves of impedance and current in an RLC parallel-resonant circuit.

Now, at resonance, XL is still equal to XC. Therefore, IL
must equal IC. Remember, the voltage is the same; the reactances are equal;
therefore, according to Ohm's law, the currents must be equal. But, don't forget, even
though the currents are equal, they are still opposites. That is, if the current is
flowing "up" in the capacitor, it is flowing "down" in the coil, and
vice versa. In effect, while the one component draws current, the other returns it to the
source. The net effect of this "give and take action" is that zero current is
drawn from the source at resonance. The two currents yield a total current of zero amperes
because they are exactly equal and opposite at resonance.

A circuit that is completed and has a voltage applied, but has zero current, must have
an INFINITE IMPEDANCE (apply Ohm's law - any voltage divided by zero yields infinity).

By now you know that we have just ignored our old friend resistance from previous
discussions. In an actual circuit, at resonance, the currents will not quite counteract
each other because each component will have different resistance. This resistance is kept
extremely low, but it is still there. The result is that a relatively small current flows
from the source at resonance instead of zero current. Therefore, a basic characteristic of
a practical parallel-LC circuit is that, at resonance, the circuit has MAXIMUM impedance
which results in MINIMUM current from the source. This current is often called line
current. This is shown by the peak of the waveform for impedance and the valley for the
line current, both occurring at fr the frequency of resonance in figure 1-11.

There is little difference between the circuit pulsed by the battery in figure 1-8 that
oscillated at its resonant (or natural) frequency, and the circuit we have just discussed.
The equal and opposite currents in the two components are the same as the currents that
charged and discharged the capacitor through the coil.

For a given source voltage, the current oscillating between the reactive parts will be
stronger at the resonant frequency of the circuit than at any other frequency. At
frequencies below resonance, capacitive current will decrease; above the resonant
frequency, inductive current will decrease. Therefore, the oscillating current (or
circulating current, as it is sometimes called), being the lesser of the two reactive
currents, will be maximum at resonance.

If you remember, the basic resonant circuit produced a "damped" wave. A
steady amplitude wave was produced by giving the circuit energy that would keep it going.
To do this, the energy had to be at the same frequency as the resonant frequency of the
circuit.

So, if the resonant frequency is "timed" right, then all other frequencies
are "out of time" and produce waves that tend to buck each other. Such
frequencies cannot produce strong oscillating currents.

In our typical parallel-resonant (LC) circuit, the line current is minimum (because the
impedance is maximum). At the same time, the internal oscillating current in the tank is
maximum. Oscillating current may be several hundred times as great as line current at
resonance.

In any case, this circuit reacts differently to the resonant frequency than it does to
all other frequencies. This makes it an effective frequency selector.

Summary of Resonance

Both series- and parallel-LC circuits discriminate between the resonant frequency and
all other frequencies by balancing an inductive reactance against an equal capacitive
reactance.

In series, these reactances create a very low impedance. In parallel, they create a
very high impedance. These characteristics govern how and where designers use resonant
circuits. A low-impedance requirement would require a series-resonant circuit. A
high-impedance requirement would require the designer to use a parallel-resonant circuit.

Tuning a Band of Frequencies

Our resonant circuits so far have been tuned to a single frequency - the resonant
frequency. This is fine if only one frequency is required. However, there are hundreds of
stations on many different frequencies.

Therefore, if we go back to our original application, that of tuning to different radio
stations, our resonant circuits are not practical. The reason is because a tuner for each
frequency would be required and this is not practical.

What is a practical solution to this problem? The answer is simple. Make either the
capacitor or the inductor variable. Remember, changing either L or C changes the resonant
frequency.

Now you know what has been happening all of these years when you "pushed" the
button or "turned" the dial. You have been changing the L or C in the tuned
circuits by the amount necessary to adjust the tuner to resonate at the desired frequency.
No matter how complex a unit, if it has LC tuners, the tuners obey these basic laws.

Q.9 What is the term for the number of times per second that tank circuit energy is
either stored in the inductor or capacitor?
Q.10 In a parallel-resonant circuit, what is the relationship between impedance and
current?
Q.11 When is line current minimum in a parallel-LC circuit?